1). File name & keyword & identifier

1. All go source codes end with. Go
2. Identifier begins with a letter or underscore, case sensitive
3. It is a special identifier to ignore the result.
4. Keep keywords

The golang keyword is as follows:

Calls to functions in the package:

a. functions in the same package can be called directly (case can be used)
b. functions in different packages are called by package name + point + function name (the initial of function name must be uppercase)

Package access control rules:

a. capitalization means that the function / variable is exportable
b. lowercase means that this function / variable is private and cannot be accessed outside the package

Example 1: write a program, for a given number n, find the combination of all the two adds equal to n.

The code is as follows:

package main

import (
    "fmt"
)

func handle(n int){
    for i := 0;i<=n;i++ {
    fmt.Printf("%d+%d=%d\n",i,n-i,n)   //  fmt.Printf()  Is formatted output
    }
}

func main(){
    handle(5)
}

Compile run:

[root@NEO project]# go build -o bin/example01_plus go_dev/day02/example01_plus/main
[root@NEO project]# bin/example01_plus 
0+5=5
1+4=5
2+3=5
3+2=5
4+1=5
5+0=5
[root@NEO project]# 

Example 2: a program contains two packages, add and main. There are two variables in the add package: Name and age. How to access Name and age in main package?

# The directory structure is as follows:
[root@NEO day02]# tree example02_call_var01
example02_call_var01
├── add
│   └── add.go
└── main
    └── main.go

2 directories, 2 files
[root@NEO day02]# 


//The sample code is as follows:
# Mode 1:
main.go Document:
package main

import (
    "go_dev/day02/example02_call_var01/add"
    "fmt"
)

func main(){
    fmt.Println("Name=",add.Name)
    fmt.Println("Age=",add.Age)
}

add.go Document:
package add

var Name string = "hello world"   // Declare a variable and initialize the value of the variable (compile time); if a string is declared but not initialized, the value of the string variable is empty
var Age int = 10        // String if declared but not initialized, the value of the string variable is 0

# The compiled run results are as follows:
[root@NEO bin]# ./example0201 
Name= hello world
Age= 10
[root@NEO bin]# 

# Mode 2:
main.go Document:
package main

import (
    "go_dev/day02/example02_call_var02/add"
    "fmt"
)

func main(){
    add.Var_test()    // Call this function first to Name and Age Initialize
    fmt.Println("Name=",add.Name)
    fmt.Println("Age=",add.Age)
}

add.go Document:
package add

var Name string
var Age int

func Var_test(){   // Capitalize the first letter
    Name = "hello world"    // go It is a compiled language. All execution statements should be put into functions (assignment)./Initialization is also an execution statement)
    Age = 10
}


# Error example:
main.go Document content:
package main

import (
    "go_dev/day02/example02_call_var03_err/add"
    "fmt"
)

function main(){
    fmt.Println("Name=",add.Name)
    fmt.Println("Age=",add.Age)
}

add.go Document content:
package add

var Name string 
var Age int    

Name = "hello world"  // go It is a compiled language. All execution statements should be put into the function as the entry.
Age = 10 

Example 3: application of package alias: develop a program that uses package alias to access functions in package

# The main.go example is as follows:
package main

import (
    a "go_dev/day02/example02_call_var01/add"    // to go_dev/day02/example02_call_var01/add This name has an alias a
    "fmt"
)

func main(){
    fmt.Println("Name=",a.Name)  // Use the package's alias to call functions in the package
    fmt.Println("Age=",a.Age)
}

Example 4: each source file can contain an init function, which is automatically called by the go runtime framework. Write a program to demonstrate this function